1. Field of the Invention
The present invention relates to a hydraulic drive system for a counterweight dolly in a counter balance type crane.
2. Description of the Prior Art
As a large-sized travelling type crane there has been known a counterbalance type crane as described, for example, in U.S. Pat. No. 3,842,984 in which a counterweight dolly is connected to the rear portion of an upper swivelling structure to improve the lifting capacity, and drive units are connected to wheels of the counterweight dolly to drive the wheels during swivelling and travelling of the upper swivelling structure of the crane body, thereby allowing the dolly to swivel around a swivelling center of the upper swivelling structure and travel simultaneously with the crane body. A counterbalance type crane which utilizes a hydraulic pressure for effecting the upper swivelling and travelling motions of the crane body has also been used.
However, in the above hydraulically driven crane, a swivelling hydraulic circuit and a travelling hydraulic circuit for the upper swivelling structure of the crane body and a dolly wheel driving hydraulic circuit are independent of each other, so the driving pressure of a swivelling hydraulic motor for the upper swivelling structure and that of a wheel driving hydraulic motor for the dolly are apt to become unbalanced due to a variation of load such as a lifting load or inclination or unevenness of the ground, particularly during swivelling motion, resulting in the fact that the upper swivelling structure and the dolly do not swivel smoothly, leading to deterioration of the craning work and a great reduction of working efficiency. If the driving force of the dolly is deficient, the swivelling hydraulic motor for the upper swivelling structure will be driven in an overloaded state, resulting in increased relief flow of the swivelling hydraulic circuit and increased power loss. Further, in order that both the upper swivelling structure and the dolly can swivel even under a variation in the driving force for swivelling the upper swivelling structure and in the driving force of the dolly, it is necessary to enlarge the size of a swivelling drive unit for the upper swivelling structure and a drive unit for the dolly. Additionally, a connecting beam which connects the upper swivelling structure and the dolly is also required to have a high strength, resulting in an increase in size and weight of the connecting beam, thus leading to an increase in cost. Thus, various problems have been involved in the conventional cranes.